The invention in general relates to fluidized-bed combustors. Particularly it relates to methods and apparatus for feeding coal and other solids into the fluidized bed and for shielding the distributor plates of such combustors thereby providing even combustion and heating throughout the system, efficient operation, and a low rate of materials failure.
In recent years the fluidized bed has become an important medium for the production of chemcial reactions such as the oxidation of coal to produce heat. The fluidized bed is produced by grinding the solid reactant, such as coal, to particle-size bits and suspending the particles in a vertically rising stream of moving gas, such as air. It is important that the air move fast enough to cause the particles to remain suspended, but not so fast that the particles are entrained, or carried along, by the air stream. Under such conditions the air-particle mixture takes on many of the properties of a fluid; for example, the "fluid" has a lower boundary at the point of the gas inlet and an upper definite and clearly marked boundary surface at which the gas disengages itself from the system. Such a fluidized bed has become important as the method of burning coal in steam generation facilitates because it provides a well-mixed medium for efficient combustion and good heat transfer.
In the conventional fluidized bed steam generation facility the fluidizing gas, which is air in this case, is introduced at the bottom of the combustion chamber through a distributor plate. The distributor plate is generally formed of high temperature metal which defines the lower boundary of the combustion chamber, and which is perforated with many holes for the passage of the air. The conventional method of feeding coat or other solids into the fluidized bed is by gravity through a standpipe. Such standpipes project into the fluidized bed where the combustion is taking place, and thus must be cooled, usually by water circulation systems. Conventionally a number of standpipes are used, placed at intervals throughout the bed for even distribution of the solid. A coal-dolomitic stone mixture may be used to capture the sulphur contained in the coal as CaSO.sub.4, a solid. Coal ash and spent dolomite are removed from the fluidized bed through water-cooled "overflow" standpipes in the bed.
The feeding of the coal-dolomite mixture (or other solid) into the fluidized bed presents significant problems in the above described steam generation facilities. If the solid is not distributed evenly throughout the bed uneven combustion rates and dolomite reaction rates can result. The overloaded sections of the bed will have a very high local heat release rate, especially when using a highly volatile coal. Such uneven heat distribution can cause materials failure in regions of excessive heating, overcooling in regions of less heat, or quenching of the dolomite reaction, all of which contribute to increased expenses and inefficiency in the system. On the otherhand, in the conventional system, if enough standpipes are used to evenly distribute the coal-dolomite feed, the bed will literally be clogged with standpipes. As a result the fluidization of the bed is decreased resulting in inefficient mixing and combustion, and in addition a significant portion of the bed volume which could be used for heat exchange (steam) tubing is usurped. As a result increased bed volume (height) may be required thereby increasing the pressure drop through the bed resulting in increased energy consumption for fluidization. Further, all the standpipes must by water-cooled, thereby reducing the amount of heat released in the combustion zone that can be used for production of steam. The uneven distribution of the dolomite throughout the system can lead to excessive discharge of sulphur into the environment.
The distributor plate in the conventional fluidized-bed steam generation facility is also a source of problems. Since the most active combustion zone is near the bottom of the bed the distributor plate may be exposed to the highest temperatures in the system. Consequently the distributor plate must be designed to allow for thermal expansion, which requires that it be "free-floating". Nevertheless because the plate may be subjected to the burden of the hot solid when the bed is slumped, the plate must be well supported. In addition, the high temperatures in the region of the plate requires that it be either cooled by large quantities of water or be well shielded. The latter method is preferred since cooling water in the region of most active combustion would be highly inefficient. The shielding material used must have openings to allow for the passage of the fluidizing air, but such openings may permit the "weeping" of hot ash and burning coal onto the distributor plate. Such hot ash and coals, being deposited on small areas by the openings in the shielding material, can cause material failure in the plate.